Well obviously sentient thought isn't what makes something human, since newborns and toddlers aren't sentient. I'm just saying they have a brain perceptive enough that extinguishing it, amounts to ending a life, as opposed to disposing of a growth of tissue.
Since we can safely assume that a sperm and an egg aren't human, and zygote isn't human, but that a new born is. We're stuck with a lot of touch questions as to how, when and why it becomes a human in between. I don't think there's a scientific way of going about this either, since it's such a nebulous and subjective concept.
The only thing I can say is that the pro life crowd has pushed too far in forbidding abortion in the incredibly early stages, since granting something as being human in the first trimester is pretty audacious, and it is coupled with a complete disregard for the woman's rights in the matter.